1. Field of the Invention
The present invention relates to digital PLL circuits. More particularly, the present invention relates to a digital PLL circuit used for reproducing an EFM (Eight to Fourteen Modulation) signal of a compact disc (CD) or a mini disc (FID).
2. Description of the Background Art
An analog PLL circuit for reproduction associated with a CD or a MD is disclosed in, for example, Japanese Patent Laying-Open Nos. 1-303630 and 3-212860. The phase difference between an input signal and a PLL clock signal generated by a PLL circuit is converted into voltage. The frequency of the PLL circuit is varied using a voltage-frequency conversion circuit to achieve synchronization. A digital PLL circuit operating under a similar principle is disclosed in Japanese Patent Laying-Open No. 3-289820, for example.
In the above-mentioned analog PLL circuit, the process of switching a gain requires the operation of switching the resistance of an operational amplifier that converts the phase difference into a voltage. This will increase the circuit complexity, and the problem of temperature characteristics is noticeable. In a conventional digital PLL circuit, a digital control oscillator is required. It was very difficult to incorporate this portion into an LSI.